Downloading the updates and reapplying them is hard as they are in repositories. These indexes need to be correct and available as well for yum to work properly otherwise you will have to resolve all dependencies manually.
Is there really no way of getting SME Server to have a internet connection at least for the duration of the updates?
Another solution would be to create a local mirror of the Centos and SME Server repositories that the server can then access, but I think that is overkill for one server.
Every major release is shipped as an ISO/CD image, you can apply those udpates if you like by upgrading from CD.